The disclosed invention is a brazing process for forming very strong joints between metals and ceramics while limiting the brazing temperature to not more than 750.degree. C., and is more particularly a process for bonding nodular cast iron to partially stabilized zirconia at brazing temperatures that do not exceed 750.degree. C.
Machine parts using ceramics in conjunction with metals have recently been developed and have advantages in high-temperature applications, since ceramics withstand very high temperatures and possess lower thermal conductivity than metals. In adiabatic turbocompound engines, for example, ceramics will permit higher operating temperatures, reduced heat loss and higher exhaust energy recovery, resulting in higher thermal efficiency. Although an engine utilizing all ceramic parts would be desirable because of the high temperature characteristics of ceramic materials, significant developmental problems must be overcome before such an engine is a practical reality. Presently, development work is being directed to the incorporation of ceramics into engines to the greatest extent possible.
Ceramics are being used as seals in space power generators and have been proposed for use in heat exchangers, but a major area of interest, and of interest with regard to this invention, is the use of ceramics in combination with metals for diesel engine parts. Insulating metal parts with ceramics inhibits heat rejection to certain diesel engine components thus reducing external cooling requirements and raising overall in-cylinder and exhaust gas temperature. A major technical problem with the adiabatic diesel engine concept has been the difficulty of reliably joining insulation ceramics to critical engine components. Of those brazing processes known in the prior art, the active filler method and the moly manganese process provided joints having inferior strengths and required excessively high temperatures. A third prior art process, the active hydride powder process, resulted in a joint of inferior strength and also failed to give consistent results.
Metal construction materials to be used in adiabatic diesel engines have strict limitations on exposure to elevated temperatures, therefore there is a need for a process that gives reliably strong joints between these metals and ceramics at low brazing temperatures. For the adiabatic diesel engine, the leading candidate for the metal construction material is nodular cast iron (NCI), while the choice for the ceramic is partially stabilized zirconia (PSZ).
NCI has a structure composed of nodules of graphite in a matrix of either all ductile ferrite, ferrite plus pearlite, or all pearlite. The pearlite, composed of a mixture of lamellar ferrite and cementite, is the constituent primarily responsible for the strength of the cast iron. For adiabatic engine pistons, nodular cast irons with a yield strength of 55 to 80 ksi will be required, which corresponds to pearlite contents of about 80 to 100%.
NCI has a lower critical transformation temperature of 723.degree. C. When cast iron is heated much above this temperature for times required for brazing (about 10 minutes), the braze joint may be damaged by a strain imparted by the transformation of the pearlite to austenite, and again further by reversible transformation strain during cool down. Another concern relative to excessive heating during brazing is the potential for weakening the nodular cast iron by spheroidization of the lamellar cementite phase of pearlite. A practical limitation on temperature when brazing piston quality NCI is thought to be about 750.degree. C.
Partially stabilized zirconia (PSZ) refers to a zirconia ceramic material stabilized by addition of other selected elements, such as Mg, Y, or Ca which maintain the zirconia primarily in the cubic phase. Whereas pure zirconia transforms from the cubic to the tetragonal to the monoclinic crystal forms on slow cooling from the melt, PSZ is maintained in the cubic form but with up to 35% tetragonal phase. Heating the PSZ at temperatures above 850.degree. C. could result in an alteration of the crystal phase structure and may cause a considerable decrease in toughness.
Therefore, there is a need to form strong, reliable joints between metals and ceramics without the requirement of extremely high brazing temperatures that could increase the likelihood of sacrificing desired physical properties of the individual components.